(1) Field of the Invention
The present invention relates to a strip-type piezoelectric resonator chip and to a method of trimming the resonator chip to adjust the frequency thereof.
Piezoelectric resonators comprise a chip of piezoelectric material, such as crystal or lithium tantalate (LiTaO.sub.3), on which a pair of electrodes is formed. When an alternating current electric field is applied between the electrodes, the piezoelectric chip is subject to the stress of a frequency equal to that of the alternating current electric field. When the frequency of the alternating current electric field coincides with the inherent resonant frequency of the piezoelectric chip, electric resonance occurs, resulting in greater oscillation of the piezoelectric chip. Oscillating devices comprising such piezoelectric resonators are widely used in oscillators or filters of communication apparatuses because of their compactness and high reliability.
(2) Description of the Prior Art
A piezoelectric chip is cut from a plate of a single crystal (a wafer) and a pair of electrode patterns is coated on the two main surfaces of the chip plate, the electrode patterns face each other through the chip plate. It is difficult to obtain a chip of a desired resonant frequency by such a process of production of the chip. Therefore, a process for adjusting the resonant frequency is required. In the process for adjusting the resonant frequency according to the prior art, metal is added to the electrode patterns by a plating method or evaporation method so as to increase the mass of the electrodes. If the plating method is used, the plating process is repeated many times and the frequency of the chip is measured until a desired frequency is obtained. If the evaporation method is used, the thickness of the electrodes is increased and the frequency of the chip which is housed within the bell jar of the evaporation device is measured until a desired frequency is obtained. However, in this prior art method, the adjusting process is troublesome and the adjustable frequency range is small.